1. Field of the Invention
This invention relates to a circuit for demodulating PSK modulated signals, and particularly to a demodulator including a differential-detection circuit for performing differential-detection of PSK modulated signals, and more particularly to an improved means for converting modulated frequency signals into phase data.
2. Description of the Related Art
Phase shift keying (hereinafter called PSK) is currently known as one method of digital modulation; it is possible to obtain a modulated signal suitable for data transfer by switching the phase of a carrier wave to multiple phases (e.g., four phases).
To demodulate such a PSK modulated signal, a demodulation method is also currently known in which the PSK modulated signal is converted in frequency into a quasi-base band and then differential-detection of the resulting signal is performed.
This quasi-base band frequency conversion is advantageous in that it is unnecessary to exactly coincide the carrier frequency of a modulated signal with the locally oscillated frequency of the demodulator, enabling accurate base band demodulation by correcting this frequency difference by a frequency error compensator circuit.
In another differential-detection method, differential detection is performed between two successive symbols (minimal unit of transfer data) to obtain a difference between symbol data during demodulating so that detection can be made with the preceding phase as a reference phase. This differential-detection method has hitherto widely been used as a differentially encoded phase shift keying (DPSK) which transfers the change of a digital signal.
However, in these conventional demodulation methods, the modulated signal is processed as a digital complex signal, thus making the construction of the demodulator circuit complicated. More particularly, the circuit for detecting phase data from the modulated signal would be made complicate.
FIG. 6 of the accompanying drawings shows a typical conventional demodulator circuit which includes a differential-detection circuit for PSK modulated signals.
As shown in FIG. 6, this demodulator circuit comprises a local oscillator 10 for oscillating a predetermined frequency, a mixer 14 for receiving a PSK modulated signal from an input terminal 12 and mixing the PSK modulated signal with a locally oscillated signal outputted from the local oscillator 10, a phase shifter 16 for shifting a locally oscillated signal by .pi./2, and a mixer 18 for receiving a PSK modulated signal from the input terminal 12 and mixing the PSK modulated signal with the output of the phase shifter 16. Thus the PSK modulated signals to be received by the mixers 14, 18 from the input terminal 12 are converted into quasi-base band signals by locally oscillated signals respectively outputted from the local oscillator 10 or by the locally oscillated signals shifted in phase by .pi./2.
The mixers 14, 18 are connected to low-pass filters 20, 22, respectively, so that harmonic components of the quasi-base band signals outputted from the mixers 14, 18 are cut off by the low-pass filters 20, 22.
Both the low-pass filters 20, 22 are connected to an A/D converter 24 where quasi-base band signals supplied via the low-pass filters 20, 22 are converted into complex amplitude data.
The A/D converter 24 is connected to a phase angle processor 26, which converts complex amplitude data into phase data and outputs the phase data.
The phase angle processor 26 is connected at one end directly to a substracter 28 and at the other end to the same substracter 28 via a 1-symbol delay circuit 30.
The 1-symbol delay circuit 30 delays phase data by 1 symbol duration. The subtractor 28 receives phase data from the phase angle processor 26 and also phase data delayed by 1-symbol delay circuit 30, subtracts the latter phase data from the from phase data, and outputs the result of subtraction as a phase difference signal.
The subtracter 28 is connected to a frequency error compensator 32 which compensates a frequency error of phase difference signal created due to the difference between transmitting carrier frequency and locally oscillated signal. The frequency error compensator 32 is connected to a decision circuit 34.
The decision circuit 34 decides 1, 0 data based on the phase difference signal whose frequency error has been compensated by the frequency error compensator 32. Therefore the demodulated signal derived from the supplied data is outputted from the decision circuit 34 to an output terminal 36.
Further, the output of the decision circuit 34 is used in compensating the frequency error in the frequency error compensator 32. The frequency error compensator 32 includes a phase error compensator circuit 38 connected to the output of the subtracter 28 for compensating a frequency error of phase difference signal, a phase error detector circuit 40 for detecting a frequency error based on both the output of the phase error compensator circuit 38 and the output of the decision circuit 34, and a averager 42 for averaging the output of the phase error detector circuit 40 and supplying an amount of compensation to the phase error compensator circuit 38.
The phase error compensator circuit 38 is an adder for adding the compensation amount, outputted from the averager 42, with the phase difference signal, outputted from the subtracter 28. When the amount of compensation is scarce or excessive in the compensator circuit 38, this lacking or excessive amount will be detected in the phase error detector circuit 40. The phase error detector circuit 40 outputs phase data for making the compensation proper, and the averager 42 averages the output of the phase error detector circuit 40 to smooth the phase data change resulting from noise and supplies the amount of compensation to the phase error compensator circuit 38.
Thus in the conventional differential-detection, modulated signals are converted into quasi-base band signals and further into complex amplitude data, whereupon the resulting data is converted into phase data.
However, the conventional demodulation methods have the following problems because the circuit for converting modulated signals into phase data is an analog circuit.
Analog elements must be used to compose the mixers and the low-pass filters so that it is difficult to integrate these components on a single semiconductor chip and hence to make them free from adjustment.
Generally, an analog-to-digital converter has been used as a digital complex signal converter; however, since the consumed electrical power of this analog-to-digital converter is large, it is difficult to save the consumed electrical power.
Further, a memory having a large storage capacitance is required to compose the phase angle processor.